Showing papers in "Monthly Notices of the Royal Astronomical Society in 1999"

Large scale bias and the peak background split

Abstract: Dark matter haloes are biased tracers of the underlying dark matter distribution. We use a simple model to provide a relation between the abundance of dark matter haloes and their spatial distribution on large scales. Our model shows that knowledge of the unconditional mass function alone is sufficient to provide an accurate estimate of the large-scale bias factor. We then use the mass function measured in numerical simulations of SCDM, OCDM and ΛCDM to compute this bias. Comparison with these simulations shows that this simple way of estimating the bias relation and its evolution is accurate for less massive haloes as well as massive ones. In particular, we show that haloes that are less/more massive than typical M* haloes at the time they form are more/less strongly clustered than is predicted by formulae based on the standard Press–Schechter mass function.

A hybrid symplectic integrator that permits close encounters between massive bodies

Abstract: Mixed-variable symplectic integrators exhibit no long-term accumulation of energy error, beyond that owing to round-off, and they are substantially faster than conventional N-body algorithms. This makes them the integrator of choice for many problems in Solar system astronomy. However, in their original formulation, they become inaccurate whenever two bodies approach one another closely. This occurs because the potential energy term for the pair undergoing the encounter becomes comparable to the terms representing the unperturbed motion in the Hamiltonian. The problem can be overcome using a hybrid method, in which the close encounter term is integrated using a conventional integrator, whilst the remaining terms are solved symplectically. In addition, using a simple separable potential technique, the hybrid scheme can be made symplectic even though it incorporates a non-symplectic component.

On the fate of gas accreting at a low rate on to a black hole

Abstract: Gas supplied conservatively to a black hole at rates well below the Eddington rate may not be able to radiate effectively and the net energy flux, including the energy transported by the viscous torque, is likely to be close to zero at all radii. This has the consequence that the gas accretes with positive energy so that it may escape. Accordingly, we propose that only a small fraction of the gas supplied actually falls on to the black hole, and that the binding energy it releases is transported radially outward by the torque so as to drive away the remainder in the form of a wind. This is a generalization of and an alternative to an `ADAF' solution. Some observational implications and possible ways to distinguish these two types of flow are briefly discussed.

Cold collapse and the core catastrophe

Abstract: We show that a universe dominated by cold dark matter fails to reproduce the rotation curves of dark matter dominated galaxies, one of the key problems that it was designed to resolve. We perform numerical simulations of the formation of dark matter haloes, each containing ≳106 particles and resolved to 0.003 times the virial radius, allowing an accurate comparison with rotation curve data. A good fit to both Galactic and cluster-sized haloes can be achieved using the density profile ρ(r)∝[(rrs)1.5(1+(rrs)1.5)]−1, where rs is a scale radius. This profile has a steeper asymptotic slope, ρ(r)∝r−1.5, and a sharper turn-over than found by lower resolution studies. The central structure of relaxed haloes that form within a hierarchical universe has a remarkably small scatter. We compare the results with a sample of dark matter dominated, low surface brightness (LSB) galaxies with circular velocities in the range 100–300 km s−1. The rotation curves of discs within cold dark matter haloes rise too steeply to match these data, which require a constant mass density in the central regions. The effects of Ωmass and Λ cannot reconcile the cold dark matter (CDM) model with data – even if we leave the concentration as a free parameter, we are unable to reproduce the observations with such a steep central density profile. It is important to confirm these results using stellar rather than H i rotation curves for LSB galaxies. We test the effects of introducing a cut-off in the power spectrum that may occur in a universe dominated by warm dark matter. In this case, haloes form by a monolithic collapse but the final density profile barely changes, demonstrating that the merger history does not play a role in determining the halo structure.

Semi-analytic modelling of galaxy formation: The local Universe

Abstract: Using semi-analytic models of galaxy formation, we investigate galaxy properties such as the Tully-Fisher relation, the B and K-band luminosity functions, cold gas contents, sizes, metallicities, and colours, and compare our results with observations of local galaxies. We investigate several different recipes for star formation and supernova feedback, including choices that are similar to the treatment in Kauffmann, White & Guiderdoni (1993) and Cole et al. (1994) as well as some new recipes. We obtain good agreement with all of the key local observations mentioned above. In particular, in our best models, we simultaneously produce good agreement with both the observed B and K-band luminosity functions and the I-band Tully-Fisher relation. Improved cooling and supernova feedback modelling, inclusion of dust extinction, and an improved Press-Schechter model all contribute to this success. We present results for several variants of the CDM family of cosmologies, and find that models with values of 0 ≃ 0.3–0.5 give the best agreement with observations.

Clustering of galaxies in a hierarchical universe - I. Methods and results at z=0

Abstract: We introduce a new technique for following the formation and evolution of galaxies in cosmological N-body simulations. Dissipationless simulations are used to track the formation and merging of dark matter haloes as a function of redshift. Simple prescriptions, taken directly from semi-analytic models of galaxy formation, are adopted for gas cooling, star formation, supernova feedback and the merging of galaxies within the haloes. This scheme enables us to explore the clustering properties of galaxies, and to investigate how selection by luminosity, colour or type influences the results. In this paper we study the properties of the galaxy distribution at z=0. These include B- and K-band luminosity functions, two-point correlation functions, pairwise peculiar velocities, cluster mass-to-light ratios, B-V colours, and star formation rates. We focus on two variants of a cold dark matter (CDM) cosmology: a high-density (Ω =1) model with shape-parameter Γ =0.21 (τ CDM), and a low-density model with Ω =0.3 and Λ =0.7 (Λ CDM). Both models are normalized to reproduce the I-band Tully--Fisher relation of Giovanelli et al. near a circular velocity of 220 km s-1. Our results depend strongly both on this normalization and on the adopted prescriptions for star formation and feedback. Very different assumptions are required to obtain an acceptable model in the two cases. For τ CDM, efficient feedback is required to suppress the growth of galaxies, particularly in low-mass field haloes. Without it, there are too many galaxies and the correlation function exhibits a strong turnover on scales below 1 Mpc. For Λ CDM, feedback must be weaker, otherwise too few L* galaxies are produced and the correlation function is too steep. Although neither model is perfect, both come close to reproducing most of the data. Given the uncertainties in modelling some of the critical physical processes, we conclude that it is not yet possible to draw firm conclusions about the values of cosmological parameters from studies of this kind. Further observational work on global star formation and feedback effects is required to narrow the range of possibilities.

SCUBA: A Common - user submillimetre camera operating on the James Clerk Maxwell telescope

Abstract: SCUBA, the Submillimetre Common-User Bolometer Array, built by the Royal Observatory Edinburgh for the James Clerk Maxwell Telescope, is the most versatile and powerful of a new generation of submillimetre cameras. It combines a sensitive dual-waveband imaging array with a three-band photometer, and is sky-background-limited by the emission from the Mauna Kea atmosphere at all observing wavelengths from 350 μμto 2 mm. The increased sensitivity and array size mean that SCUBA maps close to 10 000 times faster than its single-pixel predecessor (UKT14). SCUBA is a facility instrument, open to the world community of users, and is provided with a high level of user support. We give an overview of the instrument, describe the observing modes, user interface and performance figures on the telescope, and present a sample of the exciting new results that have revolutionized submillimetre astronomy.

Measuring and modelling the redshift evolution of clustering: the Hubble Deep Field North

Abstract: ABSTRA C T The evolution of galaxy clustering from za 0t oz . 4:5 is analysed using the angular correlation function and the photometric redshift distribution of galaxies brighter than IAB < 28:5 in the Hubble Deep Field North. The reliability of the photometric redshift estimates is discussed on the basis of the available spectroscopic redshifts, comparing different codes and investigating the effects of photometric errors. The redshift bins in which the clustering properties are measured are then optimized to take into account the uncertainties of the photometric redshifts. The results show that the comoving correlation length r0 has a small decrease in the range 0 & z & 1 followed by an increase at higher z. We compare these results with the theoretical predictions of a variety of cosmological models belonging to the general class of Cold Dark Matter scenarios, including Einstein‐de Sitter models, an open model and a flat model with non-zero cosmological constant. Comparison with the expected mass clustering evolution indicates that the observed high-redshift galaxies are biased tracers of the dark matter with an effective bias b strongly increasing with redshift. Assuming an Einstein‐de Sitter universe, we obtain b . 2: 5a tz . 2 and b . 5a tz . 4. These results support theoretical scenarios of biased galaxy formation in which the galaxies observed at high redshift are preferentially located in more massive haloes. Moreover, they suggest that the usual parameterization of the clustering evolution as jOr; zUajOr; 0UO1 1 zU 2O31eU is not a good description for any value of e . Comparison of the clustering amplitudes that we measured at z . 3 with those reported by Adelberger et al. and Giavalisco et al., based on a different selection, suggests that the clustering depends on the abundance of the objects: more abundant objects are less clustered, as expected in the paradigm of hierarchical galaxy formation. The strong clustering and high bias measured at z . 3 are consistent with the expected density of massive haloes predicted in the frame of the various cosmologies considered here. At z . 4, the strong clustering observed in the Hubble Deep Field requires a significant fraction of massive haloes to be already formed by that epoch. This feature could be a discriminant test for the cosmological parameters if confirmed by future observations.

The obscured growth of massive black holes

Abstract: The mass density of massive black holes observed locally is consistent with the hard X-ray background provided that most of the radiation produced during their growth was absorbed by surrounding gas A simple model is proposed here for the formation of galaxy bulges and central black holes in which young spheroidal galaxies have a significant distributed component of cold dusty clouds, which accounts for the absorption The central accreting black hole is assumed to emit both a quasar-like spectrum, which is absorbed by the surrounding gas, and a slow wind The power in both is less than the Eddington limit for the black hole The wind, however, exerts the most force on the gas and, as earlier suggested by Silk & Rees, when the black hole reaches a critical mass it is powerful enough to eject the cold gas from the galaxy, so terminating the growth of both black hole and galaxy In the present model this point occurs when the Thomson depth in the surrounding gas has dropped to about unity and results in the mass of the black hole being proportional to the mass of the spheroid, with the normalization agreeing with that found for local galaxies by Magorrian et al for reasonable wind parameters The model predicts a new population of hard X-ray and submm sources at redshifts above 1, which are powered by black holes in their main growth phase

Ram pressure stripping of spiral galaxies in clusters

Abstract: We use three-dimensional SPH/N-body simulations to study ram pressure stripping of gas from spiral galaxies orbiting in clusters. We find that the analytic expectation of Gunn & Gott, relating the gravitational restoring force provided by the disc to the ram pressure force, provides a good approximation to the radius at which gas will be stripped from a galaxy. However, at small radii it is also important to consider the potential provided by the bulge component. A spiral galaxy passing through the core of a rich cluster, such as Coma, will have its gaseous disc truncated to ∼4 kpc, thus losing ∼80 per cent of its diffuse gas mass. The time-scale for this to occur is a fraction of a crossing time ∼107 yr. Galaxies orbiting within poorer clusters, or inclined to the direction of motion through the intracluster medium, will lose significantly less gas. We conclude that ram pressure alone is insufficient to account for the rapid and widespread truncation of star formation observed in cluster galaxies, or the morphological transformation of Sabs to S0s that is necessary to explain the Butcher–Oemler effect.

The 1989 May outburst of the soft X‐ray transient GS 2023+338 (V404 Cyg)

Abstract: We reanalyse archival Ginga data of the soft X-ray transient source GS 20231338 covering the beginning of its 1989 May outburst. The source showed a number of rather unusual features: very high and apparently saturated luminosity, dramatic flux and spectral variability (often on ,1 s time-scale), and generally very hard spectrum, with no obvious soft thermal component characteristic for soft/high state. We describe the spectrum obtained at the maximum of flux and we demonstrate that it is very different from spectra of other soft X-ray transients at similar luminosity. We confirm previous suggestions that the dramatic variability was the result of heavy and strongly variable photoelectric absorption. We also demonstrate that for a short time the spectrum of the source did look like a typical soft/high state spectrum but that this coincided with very

The large‐scale HI structure of the Small Magellanic Cloud

Abstract: We combine new Parkes telescope observations of neutral hydrogen (Hi) in the Small Magellanic Cloud (SMC) with an Australia Telescope Compact Array (ATCA) aperture synthesis mosaic to obtain a set of images sensitive to all angular (spatial) scales between 98 arcsec (30 pc) and 4° (4 kpc). The new data are used to study the HI spatial power spectrum over a range of contiguous scale sizes wider than those previously achieved in any other galaxy, including our own. The spatial power spectrum closely obeys the relation P(k) ∝ kγ, with γ =-3.04 ± 0.02, similar to values obtained by other authors for our own Galaxy which are in the range γ =-3.0 to -2.8. This is surprising given the very different morphology, gas-richness, star-formation rate and evolution of the two systems, and may imply similar mechanisms for structure formation. One interpretation of the P(k) power-law is that the interstellar medium (ISM) of the SMC is fractal in nature, consisting of a hierarchy of HI cloud structures created, for example, by homogeneous turbulence. The projected fractal dimension of Dp=1.5 is similar to values obtained by other authors for molecular clouds in the Galaxy in the size range ∼ 0.05 to 100 pc. Such a model is consistent with a low space-filling factor for the neutral gas. A kinematic study of the HI data reveals the existence of three supergiant shells which were previously undetectable in the ATCA data alone. These shells have diameters up to 1.8 kpc and require energies (in the standard supernova-driven models) up to 2×1054 erg. The structure and evolution of the ISM in the SMC are heavily influenced by the formation of these supergiant shells.

The LX—T relation and intracluster gas fractions of X-ray clusters

Abstract: We re-examine the X-ray luminosity–temperature relation using a nearly homogeneous data set of 24 clusters selected for statistically accurate temperature measurements and absence of strong cooling flows. The data exhibit a remarkably tight power-law relation between bolometric luminosity and temperature with a slope 2.88 ± 0.15. With reasonable assumptions regarding cluster structure, we infer an upper limit on fractional variations in the intracluster gas fraction 〈(δ fgas fgas)2〉1/2≤ 15 per cent. A strictly homogeneous Ginga subset of 18 clusters places a more stringent limit of 9 per cent. Imaging data from the literature are employed to determine absolute values of fgas within spheres encompassing density contrasts δc=500 and 200 with respect to the critical density. Comparing binding mass estimates based on the virial theorem (VT) and the hydrostatic β-model (BM), we find a temperature-dependent discrepancy in fgas between the two methods caused by systematic variation of the outer slope parameter β with temperature. Mean values (for H0=50 km s−1 Mpc−1) range from f¯gas=0.10 for cool (T 4 keV) clusters using the BM at δc=200. There is evidence that cool clusters have a lower mean gas fraction than hot clusters, but it is not possible to assess the statistical significance of this effect in the present data set. The T dependence of the intracluster medium (ICM) density structure, coupled with the increase of the gas fraction with T in the VT approach, explains the steepening of the LX–T relation. The small variation about the mean gas fraction within this majority subpopulation of clusters presents an important constraint for theories of galaxy formation and supports arguments against an Einstein–de Sitter universe based on the population mean gas fraction and conventional, primordial nucleosynthesis. The apparent trend of lower gas fractions and more extended atmospheres in low-temperature systems is consistent with expectations of models incorporating the effects of galactic winds on the ICM.

The emission line—radio correlation for radio sources using the 7C Redshift Survey

Abstract: ABSTRA C T We have used narrow emission-line data from the new 7C Redshift Survey to investigate correlations between the narrow-line luminosities and the radio properties of radio galaxies and steep-spectrum quasars. The 7C Redshift Survey is a low-frequency (151 MHz) selected sample with a flux density limit about 25 times fainter than the 3CRR sample. By combining these samples, we can for the first time distinguish whether the correlations present are controlled by 151-MHz radio luminosity L151 or redshift z. We find unequivocal evidence that the dominant effect is a strong positive correlation between narrow-line luminosity LNLR and L151, of the form LNLR/ L 0:79^0:04 151 . Correlations of LNLR with redshift or radio properties, such as linear size or 151-MHz (rest frame) spectral index, are either much weaker or absent. We use simple assumptions to estimate the total bulk kinetic power Q of the jets in FR II radio sources, and confirm the underlying proportionality between jet power and narrow-line luminosity first discussed by Rawlings & Saunders. We make the assumption that the main energy input to the narrow-line region is photoionization by the quasar accretion disc, and relate Q to the disc luminosity, Qphot. We find that 0:05 & Q=Q phot & 1; so that the jet power is within about an order of magnitude of the accretion disc luminosity. Values of Q=Qphot , 1 require the volume filling factor h of the synchrotron-emitting material to be of the order of unity, and in addition require one or more of the following: (i) an important contribution to the energy budget from protons; (ii) a large reservoir of mildly relativistic electrons; and (iii) a substantial departure from the minimumenergy condition in the lobe material. The most powerful radio sources are accreting at rates close to the Eddington limit of supermassive black holes OMBH * 10 9 M(U, whilst lower power sources are accreting at sub-Eddington rates.

Radiation mechanisms and geometry of cygnus X-1 in the soft state

Abstract: We present X-ray/γ-ray spectra of Cyg X-1 observed during the transition from the hard to the soft state and in the soft state by ASCA, RXTE and CGRO/OSSE in 1996 May and June. The spectra consist of a dominant soft component below ∼2 keV and a power-law-like continuum extending to at least ∼800 keV. We interpret them as emission from an optically thick, cold accretion disc and from an optically thin, non-thermal corona above the disc. A fraction f≳0.5 of total available power is dissipated in the corona. We model the soft component by multicolour blackbody disc emission taking into account the torque-free inner-boundary condition. If the disc extends down to the minimum stable orbit, the ASCARXTE data yield the most probable black hole mass of MX≈10 M⊙ and an accretion rate, , locating Cyg X-1 in the soft state in the upper part of the stable, gas-pressure-dominated, accretion-disc solution branch. The spectrum of the corona is well modelled by repeated Compton scattering of seed photons from the disc off electrons with a hybrid, thermal/non-thermal distribution. The electron distribution can be characterized by a Maxwellian with an equilibrium temperature of kTe∼30–50 keV, a Thomson optical depth of τ∼0.3 and a quasi-power-law tail. The compactness of the corona is 2≲lh≲7, and a presence of a significant population of electron–positron pairs is ruled out. We find strong signatures of Compton reflection from a cold and ionized medium, presumably an accretion disc, with an apparent reflector solid angle, Ω/2π∼0.5–0.7. The reflected continuum is accompanied by a broad iron Kα line.

The ROSAT Brightest Cluster Sample — III. Optical spectra of the central cluster galaxies

Abstract: We present new spectra of dominant galaxies in X-ray selected clusters of galaxies, which combine with our previously published spectra to form a sample of 256 dominant galaxies in 215 clusters. 177 of the clusters are members of the ROSAT Brightest Cluster Sample (BCS; Ebeling et al. 1998), and 18 have no previous measured redshift. This is the first paper in a series correlating the properties of brightest cluster galaxies and their host clusters in the radio, optical and X-ray wavebands. 27 per cent of the central dominant galaxies have emission-line spectra, all but five with line intensity ratios typical of cooling flow nebulae. A further 6 per cent show only (NII)��6548,6584 with Hin absorption. We find no evidence for an increase in the frequency of line emission with X-ray luminosity. Purely X-ray-selected clusters at low redshift have a higher probability of containing line emission. The projected separation between the optical position of the dominant galaxy and its host cluster X- ray centroid is less for the line-emitting galaxies than for those without line emission, consistent with a closer association of the central galaxy and the gravitational centre in cooling flow clusters. The more H�-luminous galaxies have larger emission-line regions and show a higher ratio of Balmer to forbidden line emission, although there is a continuous trend of ionization behaviour across four decades in Hluminosity. Galaxies with the more luminous line emission (L(H�)> 10 41 ergs 1 ) show a significantly bluer continuum, whereas lower-luminosity and (NII)-only line emitters have continua that differ little from those of non-line emitting dominant galaxies. Values of the Balmer decrement in the more luminous systems commonly imply intrinsic reddening of E(B-V)� 0.3, and when this is corrected for, the excess blue light can be characterized by a population of massive young stars. Several of the galaxies require a large population of O stars, which also provide sufficient photoionization to produce theobserved Hluminosity. The large number of lower-mass stars relative to the O star population suggests that this anomalous population is due to a series of starbursts in the central galaxy. The lower H�-luminosity systems show a higher ionization state and few massive stars, requiring instead the introduction of a harder source of photoionization, such as turbulent mixing layers, or low-level nuclear activity. The line emission from the systems showing only (NII) is very similar to low-level LINER activity commonly found in many normal elliptical galaxies.

On the survival and destruction of spiral galaxies in clusters

Abstract: We follow the evolution of disk galaxies within a cluster that forms hierarchically in a cold dark matter N-body simulation. At a redshift z=0.5 we select several dark matter halos that have quiet merger histories and are about to enter the newly forming cluster environment. The halos are replaced with equilibrium high resolution model spirals that are constructed to represent examples of low surface brightness (LSB) and high surface brightness (HSB) galaxies. Varying the disk and halo structural parameters reveals that the response of a spiral galaxy to tidal encounters depends primarily on the potential depth of the mass distribution and the disk scale length. LSB galaxies, characterised by slowly rising rotation curves and large scale lengths, evolve dramatically under the influence of rapid encounters with substructure and strong tidal shocks from the global cluster potential --- galaxy harassment. We find that up to 90% of their stars are tidally stripped and congregate in large diffuse tails that trace the orbital path of the galaxy and form the diffuse intra-cluster light. The bound stellar remnants closely resemble the dwarf spheroidals (dE's) that populate nearby clusters. HSB galaxies are stable to the chaos of cluster formation and tidal encounters. These disks lie well within the tidally limited dark matter halos and their potentials are more concentrated. Although very few stars are stripped, the scale height of the disks increases substantially and no spiral features remain, therefore we speculate that these galaxies would be identified as S0 galaxies in present day clusters.

Hydrodynamical non-radiative accretion flows in two dimensions

Abstract: Two-dimensional (axially symmetric) numerical hydrodynamical calculations of accretion flows that cannot cool through emission of radiation are presented. The calculations begin from an equilibrium configuration consisting of a thick torus with constant specific angular momentum. Accretion is induced by the addition of a small anomalous azimuthal shear stress which is characterized by a function ν. We study the flows generated as the amplitude and form of ν are varied. A spherical polar grid which spans more than two orders of magnitude in radius is used to resolve the flow over a wide range of spatial scales. We find that convection in the inner regions produces significant outward mass motions that carry away both the energy liberated by and a large fraction of the mass participating in the accretion flow. Although the instantaneous structure of the flow is complex and dominated by convective eddies, long-time averages of the dynamical variables show remarkable correspondence to certain steady-state solutions. The two-dimensional structure of the time-averaged flow is marginally stable to the Hoiland criterion, indicating that convection is efficient. Near the equatorial plane, the radial profiles of the time-averaged variables are power laws with an index that depends on the radial scaling of the shear stress. A stress in which ν∝r1/2 recovers the widely studied self-similar solution corresponding to an ‘α-disc’. We find that, regardless of the adiabatic index of the gas, or the form or magnitude of the shear stress, the mass inflow rate is a strongly increasing function of radius, and is everywhere nearly exactly balanced by mass outflow. The net mass accretion rate through the disc is only a fraction of the rate at which mass is supplied to the inflow at large radii, and is given by the local, viscous accretion rate associated with the flow properties near the central object.

The present and future mass of the Milky Way halo

Abstract: A simple model for the Milky Way halo is presented It has a flat rotation curve in the inner regions, but the density falls off sharply beyond an outer edge This truncated, flat rotation curve (TF) model possesses a rich family of simple distribution functions which vary in velocity anisotropy The model is used to estimate the total mass of the Milky Way halo using the latest data on the motions of satellite galaxies and globular clusters at Galactocentric radii greater than 20 kpc This comprises a data set of 27 objects with known distances and radial velocities, of which six also possess measured proper motions Unlike earlier investigations, we find entirely consistent maximum likelihood solutions unaffected by the presence or absence of Leo I, provided both radial and proper motion data are used The availability of the proper motion data for the satellites is crucial as, without them, the mass estimates with and without Leo I are inconsistent at the 99 per cent confidence level All these results are derived from models in which the velocity normalization of the halo potential is taken as ∼220 km s−1 A detailed analysis of the uncertainties in our estimate is presented, including the effects of the small data set, possible incompleteness or correlations in the satellite galaxy sample and the measurement errors The most serious uncertainties come from the size of the data set, which may cause a systematic underestimate by a factor of 2, and the measurement errors, which cause a scatter in the mass of the order of a factor of 2 We conclude that the total mass of the halo is , while the mass within 50 kpc is In the near future, ground-based radial velocity surveys of samples of blue horizontal branch (BHB) stars are a valuable way to augment the sparse data set A data set of ∼200 radial velocities of BHB stars will reduce the uncertainty in the mass estimate to ∼20 per cent In the coming decade, microarcsecond astrometry will be possible with the Space Interferometry Mission (SIM) and the Global Astrometry Interferometer for Astrophysics (GAIA) satellites For example, GAIA can provide the proper motions of the distant dwarfs like Leo I to within ±15 km s−1 and the nearer dwarfs like Ursa Minor to within ±1 km s−1 This will also allow the total mass of the Milky Way to be found to ∼20 per cent SIM and GAIA will also provide an accurate estimate of the velocity normalization of the halo potential at large radii

Building up the stellar halo of the Galaxy

Abstract: We study numerical simulations of satellite galaxy disruption in a potential resembling that of the Milky Way. Our goal is to assess whether a merger origin for the stellar halo would leave observable fossil structure in the phase-space distribution of nearby stars. We show how mixing of disrupted satellites can be quantified using a coarse-grained entropy. Although after 10 Gyr few obvious asymmetries remain in the distribution of particles in configuration space, strong correlations are still present in velocity space. We give a simple analytic description of these effects, based on a linearized treatment in action-angle variables, which shows how the kinematic and density structure of the debris stream changes with time. By applying this description we find that a single dwarf elliptical-like satellite of current luminosity 10 8 L⊙ disrupted 10 Gyr ago from an orbit circulating in the inner halo (mean apocentre � 12 kpc) would contribute about � 30 kinematically cold streams with internal velocity dispersions below 5 kms −1 to the local stellar halo. If the whole stellar halo were built by such disrupted satellites, it should consist locally of 300 500 such streams. Clear detection of all these structures would require a sample of a few thousand stars with 3-D velocities accurate to better than 5 kms −1 . Even with velocity errors several times worse than this, the expected clumpiness should be quite evident. We apply our formalism to a group of stars detected near the North Galactic Pole, and derive an order of magnitude estimate for the initial properties of the progenitor system.

Cosmic confusion: degeneracies among cosmological parameters derived from measurements of microwave background anisotropies

Abstract: ABSTRA C T In the near future, observations of the cosmic microwave background (CMB) anisotropies will provide accurate determinations of many fundamental cosmological parameters. In this paper, we analyse degeneracies among cosmological parameters to illustrate some of the limitations inherent in CMB parameter estimation. For simplicity, throughout our analysis we assume a cold dark matter universe with power-law adiabatic scalar and tensor fluctuation spectra. We show that most of the variance in cosmological parameter estimates is contributed by a small number (two or three) of principal components. An exact likelihood analysis shows that the usual Fisher matrix approach can significantly overestimate the errors on cosmological parameters. We show that small correlated errors in estimates of the CMB power spectrum at levels well below the cosmic variance limits (caused, for example, by Galactic foregrounds or scanning errors) can lead to significant biases in cosmological parameters. Estimates of cosmological parameters can be improved very significantly by applying theoretical restrictions to the tensor component, and external constraints derived from more conventional astronomical observations such as measurements of the Hubble constant, Type 1a supernovae distances and observations of galaxy clustering and peculiar velocities.

MERLIN observations of relativistic ejections from GRS 1915+105

Abstract: We present high resolution MERLIN radio images of multiple relativistic ejections from GRS 1915+105 in 1997 October / November. The observations were made at a time of complex radio behaviour, corresponding to multiple optically-thin outbursts and several days of rapid radio flux oscillations. This activity followed � 20 days of a plateau state of inverted-spectrum radio emission and hard, quasi-stable X-ray emission. The radio imaging resolved four major ejection events from the system. As previously reported from earlier VLA observations of the source, we observe apparent superluminal motions resulting from intrinsically relativistic motions of the ejecta. However, our measured proper motions are significantly greater than those observed on larger angular scales with the VLA. Under the assumption of an intrinsically symmetric ejection, we can place an upper limit on the distance to GRS 1915+105 of 11.2 ± 0.8 kpc. Solutions for the velocities unambiguously require a higher intrinsic speed by about 0.1c than that derived from the earlier VLA observations, whilst the angle to the line-of-sight is not found to be significantly different. At a distance of 11 kpc, we obtain solutions of v = 0.98 +0.02 0.05c and � = 66±2 degrees. The jet also appears to be curved on a scale which corresponds to a period of around 7 days. We observe significant evolution of the linear polarisation of the approaching component, with large rotations in position angle and a general decrease in fractional polarisation. This may be due to increasing randomisation of the magnetic field within the ejected component. We do not at any time detect significant linear polarisation from the core, including periods when the flux density from this region is dominated by radio oscillations. The power input into the formation of the jet is very large, � 10 38 erg s 1 at 11 kpc for a pair plasma. If the plasma contains a cold proton for each electron, then the mass outflow rate, � 10 18 g s 1 is comparable to inflow rates

Rates of tidal disruption of stars by massive central black holes

Abstract: There is strong evidence for some kind of massive dark object in the centres of many galaxy bulges. The detection of flares from tidally disrupted stars could confirm that these objects are black holes (BHs). Here we present calculations of the stellar disruption rates in detailed dynamical models of real galaxies, taking into account the refilling of the loss cone of stars on disruptable orbits by two-body relaxation and tidal forces in non-spherical galaxies. The highest disruption rates (one star per 104 yr) occur in faint (L≲1010 L⊙) galaxies, which have steep central density cusps. More luminous galaxies are less dense and have much longer relaxation times and more massive BHs. Dwarf stars in such galaxies are swallowed whole by the BH and hence do not emit flares; giant stars could produce flares as often as every 105 yr, although the rate depends sensitively on the shape of the stellar distribution function. We discuss the possibility of detecting disruption flares in current supernova searches. The total mass of stars consumed over the lifetime of the galaxy is of the order of 106 M⊙, independent of galaxy luminosity; thus, disrupted stars may contribute significantly to the present BH mass in galaxies fainter than ∼109 L⊙.

The history of star formation in dusty galaxies

Abstract: A population of distant dusty galaxies emitting in the submillimetre waveband has recently been detected using the Submillimetre Common-User Bolometer Array (SCUBA) camera on the James Clerk Maxwell Telescope (JCMT). This population can be used to trace the amount of high-redshift star formation activity that is obscured from view in the optical waveband by dust, and so is missing from existing inventories of star formation in the distant Universe. By including this population we can construct a complete and consistent picture of the history of star formation. The evolution of obscured star formation at redshifts less than unity is constrained by mid- and far-infrared counts of dusty galaxies. Activity increases with redshift z as (1+z)γ with γ∼ 4, consistent with the form of evolution found in the optical waveband by the Canada--France Redshift Survey (CFRS) to z≤ 1. The form of evolution at higher redshifts is constrained by both faint SCUBA counts and the intensity of background radiation in the millimetre/submillimetre waveband. We find that the total amount of energy emitted by dusty galaxies is about four times greater than that inferred from rest frame ultraviolet observations, and that a larger fraction of this energy is emitted at high redshifts. The simplest explanation for these results is that a large population of luminous, strongly obscured sources at redshifts z≤ 5 is missing from optical surveys. We discuss the possible contribution of obscured active galactic nuclei to the submillimetre-wave background and counts. More accurate constraints on the history of star formation will be provided by determinations of the counts in several submillimetre wavebands and crucially by a reliable redshift distribution of the detected galaxies.

Mass flow and accretion through gaps in accretion discs

Abstract: We study the structure and dynamics of the gap created by a protoplanet in an accretion disc. The hydrodynamic equations for a flat, two-dimensional, non-self-gravitating protostellar accretion disc with an embedded, Jupiter-sized protoplanet on a circular orbit are solved. To simulate possible accretion of mass on to the protoplanet we continually remove mass from the interior of the planet's Roche lobe, which is monitored. First, it is shown that consistent results independent of numerical issues (such as boundary or initial conditions, artificial viscosity or resolution) can be obtained. Then, a detailed parameter study delineates the influence of the disc viscosity and pressure on the magnitude of the accretion rate. We find that, even after the formation of a gap in the disc, the planet is still able to accrete more mass from the disc. This accretion occurs from regions of the disc that are radially exterior and interior to the planet's orbital radius. The rate depends on the magnitude of the viscosity and vertical thickness of the disc. For a disc viscosity α=10−3 and vertical thickness H/r=0.05 we estimate the time-scale for the accumulation of one Jupiter mass to be of the order of a hundred thousand years. For a larger (smaller) viscosity and disc thickness this accretion rate is increasing (decreasing). For a very small viscosity α5×10−4 the mass accretion rate through the gap on to the planet is markedly reduced, and the corresponding accretion time-scale becomes larger than the viscous evolution time of the disc.

Chemo-spectrophotometric evolution of spiral galaxies — I. The model and the Milky Way

Abstract: ABSTRA C T The chemical and spectrophotometric evolution of spiral galaxies is investigated with detailed models, making use of up-to-date ingredients (like metallicity-dependent stellar properties) and a prescription for the star formation rate (SFR) justified both empirically and theoretically. As a first application, the model is used to describe the evolution of the Milky Way. The role of the adopted scheme of disc formation (‘inside-out’) in shaping the various chemical and colour profiles is investigated, as well as the role of extinction. It is shown that the Solar neighbourhood does not evolve like the Milky Way as a whole and that one-zone models with a non-linear SFR prescription cannot be used to study the evolution of our Galaxy. Our model average SFR is shown to match well observations of external spirals.

How to plant a merger tree

Abstract: We investigate several approaches for constructing Monte Carlo realizations of the merging history of virialized dark matter haloes (`merger trees') using the extended Press--Schechter formalism. We describe several unsuccessful methods in order to illustrate some of the difficult aspects of this problem. We develop a practical method that leads to the reconstruction of the mean quantities that can be derived from the Press--Schechter model. This method is convenient, computationally efficient, and works for any power spectrum or background cosmology. In addition, we investigate statistics that describe the distribution of the number of progenitors and their masses as a function of redshift.

The initial conditions of isolated star formation — III. Millimetre continuum mapping of pre-stellar cores

Abstract: We present the results of 1.3-mm continuum mapping observations of eight pre-stellar cores, taken with the IRAM 30-m telescope equipped with the 19-channel MPIfR bolometer array. The new 1.3-mm data, which were obtained in the `on-the-fly' mapping mode, have higher angular resolution and sensitivity than previous surveys, reaching an rms noise level of ∼ 3--9 mJy per 13-arcsec beam. The present study supports the conclusions of our previous James Clerk Maxwell Telescope (JCMT) survey and suggests that, in contrast with some theoretical predictions, most pre-stellar cores have flat inner density gradients that only approach ρ (r) ∝ r-2 beyond a few thousand au. Several of the cores have a filamentary-like morphology and are apparently fragmented in a small number (∼ 2--4) of subclumps. This implies that the initial conditions for protostellar collapse depart significantly from a singular isothermal sphere. We also note quantitative disagreement in time-scales with published ambipolar diffusion models for the quasi-static evolution of molecular cloud cores under the influence of a uniform magnetic field. We speculate that turbulent processes might help to explain our observations.

Sinking satellites and the heating of galaxy discs

Abstract: We have carried out a set of self-consistent N-body simulations to study the interaction between disc galaxies and merging satellites with the aim of determining the disc kinematical changes induced by such events. We explore a region of the parameter space embracing satellites with different masses and internal structure and orbits of various eccentricities. We find that the analytic estimates of Toth & Ostriker are high; overestimating the disc heating and thickening resulting from the accretion process by a factor of about 2--3. We find that the heating and thickening of the disc differ for satellites on prograde and retrograde orbits. The former tend to heat the stellar disc while the latter primarily produce a coherent tilt. For instance, a satellite of a Milky Way type galaxy with an initial mass of 20 per cent of that of the disc and on a retrograde orbit increases the velocity ellipsoid at the solar neighbourhood by (ΔσR, Δσπ, Δσz)⊙≈ (11,9,6) km s-1 and produces a maximum increment of the vertical scalelength and the stability parameter Q, inside the solar radius, of 300 pc and 0.8, respectively, increases of about 43 and 53 per cent. The same satellite, but on a prograde orbit, leads to changes of (ΔσR, Δσπ , Δσz)⊙≈ (22,15,12) km s-1, Δ zo,⊙≈ 550 pc and Δ Q⊙≈ 1.2. Thus, disc galaxies may accrete quite massive satellites without destroying the disc, particularly, if the orbits are retrograde. We also find that a massive bulge may play a role in reducing these effects. We have quantified the importance of the responsiveness of the halo by replacing it by a rigid potential in several simulations. In these cases, the increase of the vertical scalelength is larger by a factor of 1.5--2, indicating that a self-consistent treatment is essential for obtaining realistic results. A frequent by-product of the accretion process is the formation of weak stellar warps and asymmetric discs. Finally, we have checked how well Chandrasekhar's dynamical friction formula reproduces the sinking rates in several of our experiments. We find that it works well provided a suitable value is chosen for the Coulomb logarithm and the satellite mass is taken to be the mass still bound to the satellite at each moment.

A comparative hst imaging study of the host galaxies of radio-quiet quasars, radio-loud quasars and radio galaxies - i

Abstract: We present the first results from a major HST WFPC2 imaging study aimed at providing the first statistically meaningful comparison of the morphologies, luminosities, scalelengths and colours of the host galaxies of radio-quiet quasars, radio-loud quasars and radio galaxies. We describe the design of this study and present the images that have been obtained for the first half of our 33-source sample. We find that the hosts of all three classes of luminous AGN are massive elliptical galaxies, with scalelengths ≃10 kpc, and R−K colours consistent with mature stellar populations. Most importantly, this is first unambiguous evidence that, just like radio-loud quasars, essentially all radio-quiet quasars brighter than MR=−24 reside in massive ellipticals. This result removes the possibility that radio ‘loudness’ is directly linked to host galaxy morphology, but is however in excellent accord with the black hole/spheroid mass correlation recently highlighted by Magorrian et al. We apply the relations given by Magorrian et al. to infer the expected Eddington luminosity of the putative black hole at the centre of each of the spheroidal host galaxies we have uncovered. Comparison with the actual nuclear R-band luminosities suggests that the black holes in most of these galaxies are radiating at a few per cent of the Eddington luminosity; the brightest host galaxies in our low-z sample are capable of hosting quasars with MR≃− 28, comparable to the most luminous quasars at z≃3. Finally, we discuss our host-derived black hole masses in the context of the radio luminosity:black hole mass correlation recently uncovered for nearby galaxies by Franceschini et al., and consider the resulting implications for the physical origin of radio loudness.